[6.02] KH 15D: Unraveling the Mystery of a Peculiar Winking Star

We present the results of a high-resolution spectroscopic
monitoring program of the eclipsing pre-main-sequence star
KH 15D that reveal it to be a single-line spectroscopic
binary. We find that the radial velocity is variable with an
observed range of 11.4 km/s. The best-fitting Keplerian
model has an orbital period P = 48.38 days, which is nearly
identical to the photometric period.

We also present a historical light curve from 1967--1982,
based on our study of archival photographic plates from
Asiago Observatory. During this time, the KH 15D system
alternated periodically between bright and faint states, as
observed today. However, the bright state was 0.9 mag
brighter than the modern value, and the fractional variation
between bright and faint states (\Delta I = 0.7 mag) was
smaller than observed today (3.5 mag).

Based on these results, we find the best explanation for the
periodic dimming of KH 15D is that the binary motion carries
the visible star alternately above and below the plane of a
circumbinary disk, as recently proposed by Winn et al.
(2004) and Chiang & Murray-Clay (2004).